1. Field of the Invention
The present invention relates to a vessel for the production of molten materials. More specifically, the present invention is a vessel optimized for the handling the processing environment involved in the production of molten or liquid metals and their molding into articles of manufacture.
2. Description of the Prior Art
Metal compositions having dendritic structures at ambient temperatures conventionally have been melted and then subjected to high pressure die casting procedures. These conventional die casting procedures are limited in that they suffer from porosity, melt loss, contamination, excessive scrap, high energy consumption, lengthy duty cycles, limited die life, and restricted die configurations. Furthermore, conventional processing promotes formation of a variety of microstructural defects, such as porosity, that require subsequent, secondary processing of the articles and also result in use of conservative engineering designs with respect to mechanical properties.
Processes are known for forming metal compositions such that their microstructures, when in the semi-solid state, consist of rounded or spherical, degenerate dendritic particles surrounded by a continuous liquid phase. This is opposed to the classical equilibrium microstructure of dendrites surrounded by a continuous liquid phase. These new structures exhibit non-Newtonian viscosity, an inverse relationship between viscosity and rate of shear. The materials themselves, in this condition, are known as thixotropic materials.
One process for converting a dendritic composition into a thixotropic material involves the heating of the metal composition or alloy, hereafter just xe2x80x9calloyxe2x80x9d, to a temperature which is above its liquidus temperature and then subjecting the liquid alloy to shear or agitation as it is cooled into the region of two phase equilibria. A result of sufficient agitation during cooling is that the initially solidified phases of the alloy nucleate and grow as rounded primary particles (as opposed to interconnected dendritic particles). These primary solids are comprised of discrete degenerate dendritic spherules and are surrounded by a matrix of an unsolidified portion of the liquid metal or alloy.
Another method for forming thixotropic materials involves the heating of the alloy to a temperature at which some, but not all of the alloy is in a liquid state. The alloy may then be agitated. The agitation converts any dendritic particles into degenerate dendritic spherules. In this method, it is preferred that when initiating agitation, the semisolid metal contain more liquid phase than solid phase.
An injection molding technique using thixotropic alloys delivered in an xe2x80x9cas castxe2x80x9d state has also been seen. With this technique, the feed material is fed into a vessel where it may be further heated and at least partially melted. Next, the alloy is mechanically agitated by the action of a rotating screw, rotating plates or other means. As the material is processed, it is moved forward within the vessel. The combination of partial melting and simultaneous agitation produces a slurry of the alloy containing discrete degenerate dendritic spherical particles, or in other words, a semisolid state of the material and exhibiting thixotropic properties. The thixotropic slurry is delivered to another zone, which may be a second vessel, located adjacent a nozzle. The slurry may be prevented from leaking or drooling from the nozzle tip by controlled solidification of a solid metal plug of the material in the nozzle (by controlling the nozzle temperature). Alternatively, a mechanical or other valving scheme may be employed. The sealed nozzle provides protection to the slurry from oxidation, or the formation of oxide on the interior wall of the nozzle, that would otherwise be carried into the finished, molded part. The sealed nozzle further seals the die cavity on the injection side facilitating, if desired, the use of vacuum to evacuate the die cavity further enhancing the complexity and quality of parts so molded.
Once an appropriate amount of slurry for the production of the article has been accumulated in this zone, a piston, screw or other mechanism causes the material to be injected into the die cavity forming the desired solid article. Such casting or injection machines of the above or related varieties are herein referred to as semi-solid metal injection (SSMI) molding machines.
Currently, SSMI molding machines typically perform a substantial portion of the heating of the material in a barrel of the machine. Material enters at one section of the barrel while at a reduced temperature and is then advanced through a series of heating zones, where the temperature of the material is rapidly and, at least initially, progressively raised. The heating elements themselves, typically resistance or induction heaters, of the respective zones along the barrel may or may not be progressively hotter than the preceding heating elements. As a result, a thermal gradient exists both through the thickness of the barrel as well as along the length of the barrel.
Barrel construction for such machines has seen the barrels formed as long (up to 110 inches) and thick (outside diameters of up to 11 inches with 3 to 4 inch thick walls) monolithic cylinders. As the size and throughput capacities of these machines have increased, the length and thicknesses of the barrels have correspondingly increased. This has lead to increased thermal gradients throughout the barrels and previously unforeseen and unanticipated consequences. The primary barrel material, wrought alloy 718 (having a limiting composition of: nickel (plus cobalt), 50.00-55.00%; chromium, 17.00-21.00%; iron, bal.; columbium (plus tantalum) 4.75-5.50%, molybdenum, 2.80-3.30%; titanium, 0.65-1.15%; aluminum, 0.20-0.80; cobalt, 1.00 max.; carbon, 0.08 max.; manganese, 0.35 max.; silicon, 0.35 max.; phosphorus, 0.015 max.; sulfur, 0.015 max.; born, 0.006 max.; copper, 0.30 max. used in constructing these barrels is often in short supply and costly. Additionally, alloy 718 exhibits poor stress rupture properties, poor elongation and phase instability.
Fine grained alloy 718 of high quality is expensive and is available only as cast/wrought billet, which needs extensive boring and external machining to shape complex vessels. The scrap of alloy 718 generated by going this route can be as high as 50%. Additionally, alloy 718 is unstable at 600-700xc2x0 C., tending to transform its fine gamma double prime hardening phase to a brittle delta phase. Impact energy (Charpy V-notch) and stress rupture strength can thus degrade.
HIPPING of complex net shapes of alloy 718 is desirable to increase yield and to apply liners. However, cast/wrought alloy 718 suffers grain growth to large grains of ASTM No. 00. Impact energy (Charpy V-notch) and stress rupture strength can again degrade. Powder metal alloy 718 retains finer grain size upon HIPPING but stress rupture properties (life and ductility) still suffer severely. Furthermore, Thixomolding(copyright), semisolid metal injection molding of thixotropic alloys, is expanding into higher temperature alloys that impart additional instability to alloy 718.
In several cases, failed monolithic barrels have been analyzed and it determined that the barrels failed as a result of thermal stress and, more particularly, thermal shock in the cold or input end of the barrels. As used herein, the cold or input end of a barrel is that section or end where the material first enters into the barrel. It is in this section where the most intense thermal gradients are seen, particularly in an intermediate temperature region of the cold section, which is located downstream of where the material enters. Large grained alloy 718 has been especially prone to cracking under these high stress conditions.
During use of a SSMI molding machine, the solid material feedstock, which may be in a pellet and chip form, may be fed into the barrel while at ambient temperatures, approximately 75xc2x0 F. Being long and thick, the barrels of these molding machines are, by their very nature, thermally inefficient for heating a material introduced therein. With the influx of xe2x80x9ccoldxe2x80x9d feedstock, a region of the barrel becomes significantly cooled on its interior surface. The exterior surface of this region, however, is not substantially affected or cooled by the feedstock because the positioning of the heaters thereabout. A significant thermal gradient, measured across the barrel""s thickness, is resultingly induced in this region of the barrel. Likewise, a thermal gradient is also induced along the barrel""s length. In the region of the barrel where the highest thermal gradient has been found to develop, the barrel is heated more intensely as the heaters cycle xe2x80x9coffxe2x80x9d less frequently.
Within the barrel, shearing and moving of the feedstock longitudinally through the various heating zones of the barrel causes the feedstock""s temperature to rise, equalize at the desired level when it reaches the opposing or hot end of the barrel. At the hot end of the barrel, the processed material exhibits temperatures generally in the range of 1050-1100xc2x0 F. depending on the specific alloy being processed. For magnesium processing, the maximum temperatures to which the internal portions of the barrel is subjected are about 1180xc2x0 F. The exterior of the barrel may be heated up to 1530xc2x0 F. to achieve these temperatures.
As the feedstock is heated, the interior surface of the barrel correspondingly sees a rise in its temperature. This rise in interior surface temperatures occurs to some extent along the entire length of the barrel, including the section cooled by the influx of cold material, where its extent is lesser.
Once a sufficient amount of material is accumulated and the material exhibits its thixotropic properties, the material is injected into a die cavity having a shape conforming to the shape of the desired article of manufacture. Additional feedstock is then or continuously introduced into the cold section of the barrel, again lowering the temperature of the interior barrel surface.
As the above discussion demonstrates, the interior surface of the barrel, particularly in the region of the barrel where feed stock is introduced, experiences a cycling of its temperature during operation of the SSMI molding machine. This thermal gradient between the interior and exterior surfaces of the barrel has been seen to be as great as 350xc2x0 C.
Since the nickel content of alloy 718 is subject to be corroded by molten magnesium, currently the most commonly used thixotropic material, the vessels for producing the thixotropic alloy have been lined with a sleeve of a magnesium resistant material. Several such known materials are Stellite 12 (nominally 30Cr, 8.3W and 1.4C; Stoody-Doloro-Stellite Corp.), PM 0.80 alloy (nominally 0.8C, 27.81Cr, 4.11W and bal. Co. with 0.66N) and Nb-based alloys (such as Nb-30Ti-20W). Other molten materials, such as aluminum are also highly corrosive and errosive of materials conventionally used for components of machines for forming thixotropic materials or otherwise processing these alloys.
Obviously, where liners are used, the coefficients of expansion of the vessel and the liner must be compatible with one another for proper working of the machine. One concern with lined vessels is delamination of the liner from the remainder of the vessel or shell. Analysis of severely stressed barrels has revealed that a gap opens between the liner and the shell. This gap in turn decreases heat transfer efficiency between the liner and shell, requiring still greater temperatures to be applied to the shell and producing greater thermal gradients through the vessel.
Because of the significant cycling of the thermal gradient in the vessel, the vessel experiences thermal fatigue and shock. This can further cause cracking in the vessel and in the liner. Once the vessel liner has become cracked, processed alloy can penetrate the liner and attack the vessel. Both the cracking of the liner and the attacking of the vessel by the alloy, have previously been found to have contributed to the premature failure of the barrels.
In response to the above listed and other deficiencies, a multi-piece barrel construction has been seen with one section of the barrel designed for preparation of the thixotropic material and the other section of the barrel designed for high pressure molding requirements. These sections are referred to as the cold and hot or outlet sections of the barrel, are constructed differently and are joined together.
In a multi-piece construction, the cold section is constructed with a relatively thin (and therefore lower hoop strength) section of a material. This material, which may also be lower in cost than the material of the hot section, exhibits improved thermal conductivity and has a decreased coefficient of thermal expansion relative to the hot section material. This material also exhibits good wear and corrosion resistance to the thixotropic material intended to be processed. Several preferred materials for the cold section of the barrel are stainless steel 422, T-2888 alloy, and alloy 909, which may be lined with an Nb-based alloy (such as Nb-30Ti-20W). The hot section is constructed of a relatively thick (and therefore high hoop strength), thermal fatigue resistant, creep resistant, and thermal shock resistant material. A configuration of the hot section was to use fine grain alloy 718 with a HIPPED in lining of an Nb-based alloy, such as Nb-30Ti-20W, for lower cost and better resistance from attack by the material being processed.
A nozzle section (which is coupled to the end of the hot section opposite the cold section), may be constructed in a manner to allow residual material in the nozzle to be solidified into a sealing plug. Otherwise, the nozzle may be provided with a mechanical sealing mechanism.
While the problem of large thermal gradients in a vessel are described above with some particularity to machines and vessels for semisolid metal injection molding, the problem of large thermal gradients in a melting or pressure vessel are also seen in a wide variety of other metal molding processes and apparatuses. While the known barrel or other vessel constructions work adequately for their intended purpose, there still exists a need for an improved vessel construction that minimizes thermal stresses and that provides long life under higher service temperatures.
It is therefore a principle object of the present invention to fulfill that need by providing for an improved vessel construction for preparing molten or semi-molten metals, including, but not limited to, magnesium and aluminum.
One object of the present invention is to provide a construction having reduced thermal stresses under the above higher operating conditions.
A further object of the present invention is to provide a construction that provides a longer service life, even under higher service temperatures.
Another object of this invention is to provide a construction having deceased static and cyclic thermal stresses.
A still further object of this invention is to provide a construction that enables low cost and high production rates.
Another object of this invention is to provide one-step HIPPING of net shape components that perform with good stress rupture life, good ductility and good resistance to corrosion by liquid metals and air.
Yet another object of the present invention is to replace the shell of the barrel formed of Alloy 718 with a more stable, oxidant resistant, ductile fine grained alloy 720 or alloy of similar composition.
In achieving the above and other objects, the present invention provides a vessel for processing metallic material into a molten or semi-solid state. The vessel itself includes a body that defines a chamber into which the material is received. To receive the material, an inlet is further defined in this body. Additionally, to discharge the material from the chamber and the body, an outlet is also defined within the body. The body is further made up of a sidewall portion formed of three layers, an exterior layer, an interior layer, and an intermediate layer. The exterior layer is formed of a first material. The interior layer is formed of a second material that is different from the first material. Additionally, the interior layer defines the internal surface of the chamber mentioned above. Disposed between the interior and exterior layers is the intermediate layer. This layer is formed of a third material that is different from both the first material and the second material. The material of the intermediate layer is softer than the material of both the exterior layer and the interior layer and as such, it minimizes the thermal gradient experienced through the thickness of the vessel as well as along the length of the vessel. It bonds to the interior and exterior layers and blocks any liquid metal corrosion attack of the outer layer. By reducing this thermal gradient, stresses within the vessel are also reduced and a corresponding increase in the life of the vessel results.
Modification of the hardening mechanism of alloy 718 can stabilize the hardening mechanism and eliminate the delta phase precipitation. This affords Ni base superalloys greater strength at 600-750xc2x0 C. with long-time life and retention of ductility. These alloys, e.g. alloy 720, use lower Nb and higher Ti+Al to attain a stable gamma prime phase. Furthermore, these preferred alloys can be HIPPED at high temperatures (e.g. 1150xc2x0 C.) without the pronounced grain growth seen in cast/wrought alloy 718 and degradation of properties seen in powder metallurgy alloy 718 from grain boundary precipitates. Thus, 3 layer constructions of super-alloy barrel, bond layer and liner can be HIPPED in one step to make net shapes that require little machining and material loss, hence lower cost.
Inserts for hot sprues and hot runners and shot sleeves can be constructed in the same 3 layer format.